We propose to continue studies of certain transport characteristics of the immature mammalian blood-gas barrier. The purpose of this work is to document functional change in the pulmonary microvascular endothelium and alveolar epithelium in rabbits ranging in age from prenatal to adult. We are particularly interested in the transport of fluid and lipid-insoluble solutes, and ultimately intend to quantify developmental changes in terms of equivalent pore radii for these two barriers. We shall continue to use the isolated, Ringer-perfused lung preparation which we currently employ. Using a tissue sampling technique we shall measure the pulmonary microvascular endothelium permeability to several macromolecules. We shall use several molecules in a single experiment, express our results as permeability ratios, and from this estimate equivalent pore radii at prenatal, newborn, adult and intermediate ages. The results of these studies will be used to verify and extend previous work using an osmotic transient technique. We shall also measure the alveolar epithelium permeability to several smaller lipid-insoluble solutes using previously established techniques in isolated lungs. An equivalent pore radius for this barrier will also be calculated and compared for several developmental ages. This work should provide new information about functional blood-gas barrier changes during lung development. Our results will hopefully provide a quantitative basis for understanding normal and abnormal fluid and protein movements in immature lungs.